The present invention relates to the structure and method of installation of a deep foundation to support large diameter, tall towers in a wide range of soil conditions. The inventive foundation exhibits high tension, high compression and high resistance to overturning moments acting on the supported tower. The inventive foundation depends, on below ground embedded tension/compression components that each terminate in a distal formation of enhanced bearing, tension and compression capacity. The embedded component may be driven into position, eliminating the effort, expense and time for deep wide-area site excavations and dewatering. This inventive deep foundation is especially suitable for support of tall tubular towers, such as wind turbines.
Various installations require foundations to support tall large diameter tubular towers. Such installations include power generating wind turbines, power line towers, transmission and communication towers, fluid tank (water) towers, emission stacks and similar tower structures. The towers are typically round, fabricated from welded plates, and have a horizontal circular flange plate (or cap) for anchor bolt connection at the ground level base of the tower. The towers are generally steel, with a relatively large base diameter (about 8 feet or greater). The towers may be more than about 100 feet tall and subject to very large overturning moments with relatively moderate lateral loads and small vertical loads.
Conventional foundations for such towers have required large embedments, such as large diameter piers, cylinders or gravity-spread foundations. Conventional foundations for wind turbine towers have become very large and expensive, as the size of the wind turbines and the height of the towers has increased. Conventional gravity-spread foundations may require a below ground maximum diameter of about 40 feet to bout 50 feet to support a tower pedestal of about 15 feet in diameter.
Conventional gravity-spread foundations require a substantial below ground mass and depend, largely, on the vertical or lateral bearing capacity of soils near the surface. If the bearing capacity of the deeper soils or the weight of the upper level mass of soil is needed, large deep wide-area excavations are required. As used herein, the term xe2x80x9cexcavationxe2x80x9d refers to cutting and digging a large diameter cavity and to scooping out and removing the soil generally to the full depth and diameter of the hollowed-out cavity. Deeper larger diameter foundations spread the load of the aboveground structure, but add to the cost, time and difficulty of installation. High groundwater can require dewatering operations that complicate and increase the cost of deep, wide area excavations for conventional gravity-spread foundations.
U.S. Pat. No. 5,586,417, issued Dec. 24, 1996, entitled Tensionless Pier Foundation and U.S. Pat. No. 5,826,387, issued Oct. 27, 1998, entitled Pier Foundation under High Unit Compression each describe large diameter below ground foundations of poured-on-site cementitious monolithic construction. The foundations described in these two patents currently are a foundation of choice for wind turbine installations. The foundations of these patents use two concentric corrugated cylinders, requiring extensive deep wide area site excavation and subsequent back filling and compacting. These patents require high compression of the anchor bolts on the poured foundations and large soil mass to achieve resistance to large overturning moments on the supported tower. The present inventive foundation answers a need to avoid labor-intensive deep, wide area site excavation, controlled replacement of soil and expensive fabricated steel matrix reinforcement. The inventive foundation requires a smaller amount of concrete than conventional foundations, such as those described in these two patents.
An embedded high-tension, high-compression foundation for an above ground tower comprises a ground level cap, attachments for securing the tower to the cap and belowground embedded tension/compression components. The tension/compression components are each secured to the cap and each terminate distally with a below ground anchoring structure. The anchoring structure provides embedded below ground tension retention of the components within the deep level soil and/or rock mass. The components extend to deep, high-strength soil layers. The components are embedded without the need for deep wide area site excavation. The foundation of this invention requires only shallow excavation near the surface for placement of the cap. The components with their distal anchoring structure provide exceptional bearing and tension capacity, and high resistance to overturning moment forces acting on the supported above-ground structure.
The cap may be steel reinforced concrete. The attachments for securing the tower to the cap may be conventional anchor bolts or a flange structure for bolt attachment, such as a steel embedment with a circular flange plate for bolt attachment. As used herein, the terms xe2x80x9ctension/compression component,xe2x80x9d xe2x80x9cembedded component,xe2x80x9d or simply xe2x80x9ccomponentxe2x80x9d refer to a below ground embedded element that extends to a desired below ground depth and terminates in a distal formation contributing enhanced bearing, tension and compression capacity to the component and the supported above-ground structure. Non-limiting but illustrative examples of such components include piles with distal end helical fins, piles with a distal end grouted soil or rock anchor, piles with distal end helical soil or rock anchors, caissons with a distal belled section, caissons with a distal end grouted soil or rock anchor, caissons with distal end helical screw anchors or any combinations thereof. A pile with distal end helical fins is a pile with one or more fins welded or otherwise formed at the pile distal end in a helical or spiral configuration. Such a pile has been referred to as a xe2x80x9cspin-fin pile.xe2x80x9d The distal formation contributing enhanced bearing, tension and compression capacity to the component may be preformed or may be formed in place. Thus, a suitable tension/compression component may be structured as follows. A pile constructed with side apertures adjacent the distal end of the pile is driven to its desired belowground position. To prevent occlusion of the pile lumen with rock and/or soil debris during driving, the pile may have a suitable closed distal end, such as a threaded or non-threaded point or auger tip. A suitable resinous fluid is introduced to the pile interior to permeate through the apertures and bond with the surrounding deep soil. If the components are hollow, they may then be filled, for example, with concrete. The components may be straight or tapered. If the components are tapered, they taper from a larger cross-sectional area near the soil surface to a smaller cross-sectional area at deep soil areas.
This invention is also a method of constructing an embedded high-tension, high-compression foundation for an above ground tower. A minimal ground-level excavation is established for the cap. Tension/compression components are embedded into deep, high-strength soil and/or rock layers to provide exceptional bearing and tension capacity. The distal anchoring means of the embedded components provide high-tension retention within the mass of deep, high-strength soil and or rock layers. The components are embedded by driving, augering, drilling, and the like, without the need for deep below ground wide-area excavation.
As used herein, the term xe2x80x9cembeddingxe2x80x9d refers to a process for positioning the component by locating the component at ground level above its desired final location and imparting impetus to forcibly plunge the component through the intervening soil and/or rock formations. The impetus and/or the shape of the component (e.g., a spin fin pile) may cause the component to rotate slightly while advancing to its desired final location. xe2x80x9cEmbeddingxe2x80x9d also refers to a process of positioning the component by establishing a hole in the intervening soil and/or rock formations of essentially the same or only slightly larger diameter than the component, so that the embedded component may be advanced or lowered into its desired final location within the hole. Alternatively, the component may be formed in place within the established hole. The process of establishing a hole may be by piercing, sinking or penetrating a hole sized to or only slightly larger than the component, in essentially a straight line. Typically, when the component is based on a pile, embedding may be by imparting impetus to plunge the component forcibly to its desired position. When the component is based on a caisson, embedding may be by establishing a hole of essentially the same or only slightly larger diameter than the caisson and forming the caisson in place.
The cap is then formed and the components are secured to the cap by any suitable method. Forming the cap may comprise placing formwork for the cap, including reinforcement and means for attachment of the tower, placing concrete in the formwork, stripping the formwork, and backfilling and compacting around the cap. The tower is then attached to the cap by any suitable method. The cap may be a horizontal circular flange plate. The reinforcement may be steel. The attachments may be anchor bolts. Spin fin piles and other components that are pipe piles (such as the straight or tapered piles with distal end helical rock or anchor screws and piles with a distal end grouted soil or rock anchor) may be embedded by driving, drilling or augering. The pipe pile may be driven with or without an end plate. If the tension/compression components are caissons, the only formation of a hole essentially sized to the caisson is required and the caisson is formed in its embedded position. Auguring or similar drilling methods may form a hole for formation and positioning of the caisson. The tension/compression components may be filled, for example, with concrete. The tension/compression components may be battered outwardly from the cap and tower.